1. Field of the Invention
The present invention relates generally to a method of heat treatment of a substrate, and more specifically to a method of heat treatment in a heat treatment unit in a substrate treatment apparatus including a plurality of substrate treatment units including the heat treatment unit and a self-propelled substrate conveying robot for traveling around the treatment units to convey a substrate thereto.
2. Description of the Related Art
Heat treatment of semiconductor substrates (referred to simply as "substrate" hereinafter) plays an important role in manufacturing a semiconductor device. Heat-treating of photoresist applied on the surface of a substrate in the process of photolithography for forming wirings into a desired form on the surface of the substrate, for example, has significant effects on the accuracy of wiring patterning. Therefore, various kinds of apparatus have been employed for various kinds of treatment including heat-treatment of a large number of substrates efficiently.
Such a substrate treatment apparatus is for example used for washing a substrate, applying photoresist onto the surface of a substrate, developing and heating/drying the photoresist and the like. The substrate treatment apparatus therefore includes a plurality of substrate treatment units and heat treatment units, and a single self-propelled substrate conveying robot for conveying sequentially substrates to the substrate treatment units and the heat treatment units so that the substrate is sequentially conveyed to each unit at a cycle time T. The cycle time means a time period required for the robot to move round back to the same treatment unit, and a substrate disposed in each of the treatment units can be taken out only at this cycle time.
Heating a substrate for less than a prescribed heating time period t does not allow sufficient heat treatment of the substrate, resulting in for example a problem that the photoresist is not dried, and on the other hand heating the substrate for more than the prescribed heating time period t in the heat treatment unit causes overbaking, thereby deteriorating the sensitivity of the photoresist. Preferably, the substrate should be heated in a time period which corresponds to the prescribed heating time period. There will be no problem if the prescribed heating time corresponds to the cycle time in order to meet this end, but this is not usually the case in practice.
This is caused by the following reasons. The prescribed heating time period is determined by restrictions associated with the process, such as the chemical properties, film thickness, etc. of the photoresist. On the other hand, the cycle time period is determined by the mechanical restrictions of the self-propelled substrate conveying robot such as time required for the self-propelled substrate conveying robot to dispose and take out a substrate to and from each of the treatment units, a speed at which the robot move around the treatment units, etc. The two time periods are determined by the factors irrelevant to each other as mentioned above and therefore can not be expected to be the same.
However, if the prescribed heating time period is longer than the cycle time period, there will not be any problem because the cycle time can be set to be longer on purpose, and therefore this is not what the present invention is directed to.
There will be a problem, however, if the cycle time period is longer than the prescribed heating time period. In this case, a substrate disposed in the heat treatment unit cannot be taken out of the unit by the self-propelled substrate conveying robot until the end of the cycle time period after the prescribed heating time period elapses, and therefore it can not be helped that the substrate stays in the heat treatment unit. In that case, the substrate stays in the heat treatment units in an inappropriately long time period and can be subject to a danger of being overbaked. Danger of this kind should be eliminated.
In order to achieve this purpose, a substrate heating apparatus as shown in FIG. 1 for example, is disclosed in Japanese Patent Publication No. 1-49010. Now, referring to FIG. 1, the substrate heating apparatus includes a hot plate 124 having cold air outlets 124a and through-holes 126a; substrate supporting pins 126 for supporting at their tip a substrate W and being moved up and down in the through-holes 126a by an air cylinder, etc. (not shown); a cold air supply device (not shown) for supplying a cold air from the cold air outlets 124a toward the bottom of the substrate W in a desired timing; and a controller (not shown) for controlling the air cylinder for moving the substrate supporting pin 126 up and down, the cold air supply device and the hot plate 124.
The substrate heating apparatus shown in FIG. 1 operates as follows. Referring to FIG. 1(A), the substrate supporting pins 126 are drawn into the through-holes 126a. When the substrate W is placed on the hot plate 124, heating of the substrate W by the hot plate 124 is initiated. The heating continues for a prescribed time period t.
After the time t elapses, the substrate supporting pins 126 elevate, and the substrate W is separated from the surface of the hot plate 124. Cold air is supplied from the cold air outlet 124a to the bottom of the substrate W at this time.
In the above-described conventional apparatus, the hot plate 124 is selectively cooled in the vicinity of the outlet 124a when the cold air passes through the outlet 124a at the hot plate 124, and the blown cold air diffuses unevenly on the surface of the hot plate 124 to cool the surface of the hot plate 124 unevenly, resulting in disparity in the temperature distribution of the hot plate. The distribution of the temperature at which a succeeding substrate is heated will therefore be uneven, deteriorating the quality of heat treatment applied to the substrate. Also, the cost for providing the cold air outlets 124a at the hot plate 124 has been high.
These problems have been obstacles to the heat treatment of substrates while preventing the overbaking and the unevenness in the treatment as well.